In connection with popularization of high-power light sources, there is a growing need for a protective device capable of protecting an optical system from an excessive light input. An optical fuse is one type of such protective devices. The optical fuse has a function of cutting off an optical line or restricting an amount of light passing therethrough in response to an excessive input, in a manner similar to a fuse incorporated in most electric products.
Previously proposed optical fuses have been designed such that a medium for causing a light loss is inserted into an optical line, wherein the medium includes a material in which an irreversible change is to be induced in response to excessive light entered therein (see, for example, the following Patent Publications 1 to 3). In this structure, the medium is arranged at a position where entire light traveling along the optical line passes therethrough, so that the optical fuse function is achieved based on an increase in insertion loss of the medium to be caused by an excessive light input. This structure has a disadvantage about inherently large insertion loss due to relatively high light-absorbance capacity of the irreversibly-changeable material.
An optical limiter is known as a device with a similar structure to that of the optical fuse. The optical limiter is designed to attenuate light in response to an excessive light input and allow only a given amount of light to pass therethrough, wherein an insertion loss is increased in response to an excessive input by utilizing a reversible phenomenon (see, for example, the following Patent Publications 4 and 5). As with the structure of the optical fuse, the optical limiter has a medium arranged at a position where entire light traveling along an optical line passes therethrough, and a material having a nonlinear optical effect or a light-absorbing material is used in the medium. Thus, this structure also has a disadvantage about inherently large insertion loss.
Another disadvantage of the conventional optical fuse is that a light output cannot be completely cut off because a light input port is optically coupled to a light output port even after the irreversible change is induced. Specifically, when an excessive light input continues for a long period of time, even attenuated light can keep providing some light output to result in continuous supply of light energy to a downstream side of the optical fuse.
Additionally, other related techniques for achieving an optical limiter will be described below together with a discussion thereabout. There has been known one technique in which two light waveguides are arranged in opposed relation to one another without interposing therebetween the medium used in the aforementioned techniques, and then moved apart from one another in response to increase in intensity of light traveling along an optical line so as to restrict an amount of light passing across the two light waveguides. As disclosed, for example, in the following Patent Publication 6, a core and clad of an optical waveguide are made of materials different in temperature coefficient of thermal expansion, and the core is arranged at a position displaced from the center of the clad, so that the optical limiter function is achieved by utilizing a phenomenon that the optical waveguide is deformed by heat generated therein due to an increase in intensity of light traveling along an optical line.
Alternatively, the core and clad are made of materials different in temperature coefficient of refractive index, and the optical wave guide is arranged in a bent state, so that the optical limiter function is achieved by utilizing a phenomenon that a light confinement effect of the optical waveguide is lowered by heat generated therein due to an increase in intensity of light traveling along an optical line, whereby an amount of light leakage is increased. While the above phenomenon utilizing a difference between respective temperature coefficients of the core and clad materials is based on a reversible change, it is readily understood that an optical fuse can be achieved by replacing such phenomenon with a phenomenon inducing an irreversible change. An amount of deformation or light leakage of the optical waveguide in this technique is sufficient to increase a desired insertion loss but not to eliminate an optical coupling. Thus, this technique also has a disadvantage about difficulty in completely cutting off a light output.
As above, the conventional optical fuses have problems about relatively large insertion loss in a normal state, and undesirable continuation of an optical coupling in a state after an irreversible change is induced. While KiloLambda Technologies, Ltd., USA, announced a sales plan of a low-loss optical fuse on Mar. 17, 2003, its technical content has not been known so far.
Patent Publication 1: Japanese Patent Laid-Open Publication No. 2002-221740 (Optical Fuse Device)
Patent Publication 2: Japanese Patent No. 3169885 (Optical Fuse)
Patent Publication 3: Japanese Patent Laid-Open Publication No. 11-281842 (Optical Fuse, Optical Fuse Composite Structure and Optical Fuse Device comprising Same)
Patent Publication 4: Japanese Patent Laid-Open Publication No. 11-109427 (Optical Waveguide Type Light-Intensity Attenuation Element)
Patent Publication 5: U.S. Pat. No. 6,415,075 (Potothermal Optical Signallimiter)
Patent Publication 6: WO 03/058338 (Optical Limiter)